r"""File-like objects that read from or write to a string buffer. This implements (nearly) all stdio methods. f = StringIO() # ready for writing f = StringIO(buf) # ready for reading f.close() # explicitly release resources held flag = f.isatty() # always false pos = f.tell() # get current position f.seek(pos) # set current position f.seek(pos, mode) # mode 0: absolute; 1: relative; 2: relative to EOF buf = f.read() # read until EOF buf = f.read(n) # read up to n bytes buf = f.readline() # read until end of line ('\n') or EOF list = f.readlines()# list of f.readline() results until EOF f.truncate([size]) # truncate file at to at most size (default: current pos) f.write(buf) # write at current position f.writelines(list) # for line in list: f.write(line) f.getvalue() # return whole file's contents as a string Notes: - Using a real file is often faster (but less convenient). - There's also a much faster implementation in C, called cStringIO, but it's not subclassable. - fileno() is left unimplemented so that code which uses it triggers an exception early. - Seeking far beyond EOF and then writing will insert real null bytes that occupy space in the buffer. - There's a simple test set (see end of this file). """ try: from errno import EINVAL except ImportError: EINVAL = 22 __all__ = ["StringIO"] def _complain_ifclosed(closed): if closed: raise ValueError, "I/O operation on closed file" class StringIO: """class StringIO([buffer]) When a StringIO object is created, it can be initialized to an existing string by passing the string to the constructor. If no string is given, the StringIO will start empty. The StringIO object can accept either Unicode or 8-bit strings, but mixing the two may take some care. If both are used, 8-bit strings that cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause a UnicodeError to be raised when getvalue() is called. """ def __init__(self, buf = ''): # Force self.buf to be a string or unicode if not isinstance(buf, basestring): buf = str(buf) self.buf = buf self.len = len(buf) self.buflist = [] self.pos = 0 self.closed = False self.softspace = 0 def __iter__(self): return self def next(self): """A file object is its own iterator, for example iter(f) returns f (unless f is closed). When a file is used as an iterator, typically in a for loop (for example, for line in f: print line), the next() method is called repeatedly. This method returns the next input line, or raises StopIteration when EOF is hit. """ _complain_ifclosed(self.closed) r = self.readline() if not r: raise StopIteration return r def close(self): """Free the memory buffer. """ if not self.closed: self.closed = True del self.buf, self.pos def isatty(self): """Returns False because StringIO objects are not connected to a tty-like device. """ _complain_ifclosed(self.closed) return False def seek(self, pos, mode = 0): """Set the file's current position. The mode argument is optional and defaults to 0 (absolute file positioning); other values are 1 (seek relative to the current position) and 2 (seek relative to the file's end). There is no return value. """ _complain_ifclosed(self.closed) if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] if mode == 1: pos += self.pos elif mode == 2: pos += self.len self.pos = max(0, pos) def tell(self): """Return the file's current position.""" _complain_ifclosed(self.closed) return self.pos def read(self, n = -1): """Read at most size bytes from the file (less if the read hits EOF before obtaining size bytes). If the size argument is negative or omitted, read all data until EOF is reached. The bytes are returned as a string object. An empty string is returned when EOF is encountered immediately. """ _complain_ifclosed(self.closed) if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] if n is None or n < 0: newpos = self.len else: newpos = min(self.pos+n, self.len) r = self.buf[self.pos:newpos] self.pos = newpos return r def readline(self, length=None): r"""Read one entire line from the file. A trailing newline character is kept in the string (but may be absent when a file ends with an incomplete line). If the size argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned. An empty string is returned only when EOF is encountered immediately. Note: Unlike stdio's fgets(), the returned string contains null characters ('\0') if they occurred in the input. """ _complain_ifclosed(self.closed) if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] i = self.buf.find('\n', self.pos) if i < 0: newpos = self.len else: newpos = i+1 if length is not None and length >= 0: if self.pos + length < newpos: newpos = self.pos + length r = self.buf[self.pos:newpos] self.pos = newpos return r def readlines(self, sizehint = 0): """Read until EOF using readline() and return a list containing the lines thus read. If the optional sizehint argument is present, instead of reading up to EOF, whole lines totalling approximately sizehint bytes (or more to accommodate a final whole line). """ total = 0 lines = [] line = self.readline() while line: lines.append(line) total += len(line) if 0 < sizehint <= total: break line = self.readline() return lines def truncate(self, size=None): """Truncate the file's size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position. The current file position is not changed unless the position is beyond the new file size. If the specified size exceeds the file's current size, the file remains unchanged. """ _complain_ifclosed(self.closed) if size is None: size = self.pos elif size < 0: raise IOError(EINVAL, "Negative size not allowed") elif size < self.pos: self.pos = size self.buf = self.getvalue()[:size] self.len = size def write(self, s): """Write a string to the file. There is no return value. """ _complain_ifclosed(self.closed) if not s: return # Force s to be a string or unicode if not isinstance(s, basestring): s = str(s) spos = self.pos slen = self.len if spos == slen: self.buflist.append(s) self.len = self.pos = spos + len(s) return if spos > slen: self.buflist.append('\0'*(spos - slen)) slen = spos newpos = spos + len(s) if spos < slen: if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [self.buf[:spos], s, self.buf[newpos:]] self.buf = '' if newpos > slen: slen = newpos else: self.buflist.append(s) slen = newpos self.len = slen self.pos = newpos def writelines(self, iterable): """Write a sequence of strings to the file. The sequence can be any iterable object producing strings, typically a list of strings. There is no return value. (The name is intended to match readlines(); writelines() does not add line separators.) """ write = self.write for line in iterable: write(line) def flush(self): """Flush the internal buffer """ _complain_ifclosed(self.closed) def getvalue(self): """ Retrieve the entire contents of the "file" at any time before the StringIO object's close() method is called. The StringIO object can accept either Unicode or 8-bit strings, but mixing the two may take some care. If both are used, 8-bit strings that cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause a UnicodeError to be raised when getvalue() is called. """ _complain_ifclosed(self.closed) if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] return self.buf # A little test suite def test(): import sys if sys.argv[1:]: file = sys.argv[1] else: file = '/etc/passwd' lines = open(file, 'r').readlines() text = open(file, 'r').read() f = StringIO() for line in lines[:-2]: f.write(line) f.writelines(lines[-2:]) if f.getvalue() != text: raise RuntimeError, 'write failed' length = f.tell() print 'File length =', length f.seek(len(lines[0])) f.write(lines[1]) f.seek(0) print 'First line =', repr(f.readline()) print 'Position =', f.tell() line = f.readline() print 'Second line =', repr(line) f.seek(-len(line), 1) line2 = f.read(len(line)) if line != line2: raise RuntimeError, 'bad result after seek back' f.seek(len(line2), 1) list = f.readlines() line = list[-1] f.seek(f.tell() - len(line)) line2 = f.read() if line != line2: raise RuntimeError, 'bad result after seek back from EOF' print 'Read', len(list), 'more lines' print 'File length =', f.tell() if f.tell() != length: raise RuntimeError, 'bad length' f.truncate(length/2) f.seek(0, 2) print 'Truncated length =', f.tell() if f.tell() != length/2: raise RuntimeError, 'truncate did not adjust length' f.close() if __name__ == '__main__': test()